This invention relates to a process for preparing 2-carboxyalkyl(phenyl)-phosphinic acid. In a specific aspect, this invention relates to an improved hydrolysis process. In a more specific aspect, this invention relates to a process for preparing 2-carboxyethyl(phenyl)-phosphinic acid using an improved hydrolysis process.
2-Carboxyethyl(phenyl)phosphinic acid, a flame retardant additive for polymers such as polyesters, has been prepared by first reacting acrylic acid with dichloro(phenyl)phosphine. This reaction mixture was then subjected to a separate hydrolysis step to obtain the 2-carboxyethyl(phenyl)phosphinic acid.
U.S. Pat. No. 4,081,463 (Birum et al.) discloses a process in which a 25-45% molar excess of acrylic acid was used in the reaction of acrylic acid with dichloro(phenyl)phosphine. For the hydrolysis step, the '463 patent discloses adding the undiluted reaction product of acrylic acid and dichloro(phenyl)phosphine to at least enough water to complete hydrolysis, with it being advantageous to use a significant excess of water to aid stirring and temperature control. In fact, use of a 5 to 15 molar excess of water is taught as being convenient. This molar excess of water is equivalent to conducting the hydrolysis using 12 to 32 moles water per mole of dichloro(phenyl)phosphine charged to the reaction of acrylic acid with dichloro(phenyl)phosphine. Conducting the hydrolysis in this manner produces 2-carboxyethyl(phenyl)phosphinic acid as a fine white solid.
U.S. Pat. No. 4,769,182 (Hazen) discloses a process in which a 0-20% molar excess of acrylic acid was used in the reaction of acrylic acid with dichloro(phenyl)phosphine. For the hydrolysis step, the '182 patent discloses that typical hydrolysis conditions for acid chlorides and similar water-reactive species are used, with a "drowning" technique being preferred. The drowning technique involves the use of significant excess water, i.e. 25.7 moles water per mole of dichloro(phenyl)phosphine charged to the reaction of acrylic acid with dichloro(phenyl)phosphine. Conducting the hydrolysis in this manner produces 2-carboxyethyl(phenyl)phosphinic acid as a white microcrystalline powder.
It is desirable to produce 2-carboxyalkyl(phenyl)phosphinic acid with improved product recovery and washing resulting in improved product quality.
It has now been surprisingly discovered that conducting the hydrolysis with significantly less water present than suggested by the prior art produces 2-carboxyalkyl(phenyl)phosphinic acid having better filterability than obtained using the prior art processes.